Typically, cell number is determined, directly, by microscopic or electronic enumeration, or indirectly, by the use of chromogenic dyes, incorporation of radioactive precursors or measurement of metabolic activity of cellular enzymes. These methods are often insensitive, labor intensive, or hazardous (.sup.3 H-thymidine incorporation).
A recent colorimetric assay is based on the cellular conversion of a tetrazolium salt into a blue formazan product that is detected using a ELISA plate reader. (Mossmann T., J. Immunol, Meth. 65:55-63 (1983)
Despite numerous attempts to reduce several technical problems related to this method (e.g., protein precipitation and incomplete solubilization of the formazan product), the assay is still time consuming (4 hours incubation at 37.degree. C.) and still requires the use of highly toxic materials (e.g., dimethyl formamide and thiazoyl blue).